1. Field of the Invention
The present invention relates to a vibration driven power generation element and to a method of manufacture thereof.
2. Description of Related Art
Power generation elements that convert vibrational energy into electrical energy of a type that incorporates an electret with two mutually opposing planar comb tooth type electrodes are per se known. Almost all of these have a structure in which comb teeth are provided to two mutually opposing rectangular regions (for example, refer to Japanese Laid-Open Patent Publication 2010-136598) or circular regions (for example, refer to Japanese Laid-Open Patent Publication 2011-072070).
With a power generation element utilizing a structure in which an electret is provided with such planar comb tooth type electrodes, it is necessary to make the electrode area large in order to increase the power generation capacity, and accordingly it has been difficult to manufacture a power generation element that is compact and whose output is large.
In recent years, a compact power generation element has been manufactured in which the electrostatic capacitance between the two mutually opposing electrodes is increased by applying MEMS technology, and by forming these comb tooth electrodes in a three-dimensional structure. For example, mutually opposing three-dimensional comb tooth electrodes of this type are used in the power generation element disclosed in Japanese Laid-Open Patent Publication 2010-011547, and an electret that is provided in the vicinity of the comb tooth electrodes is employed as a means for applying voltage between the electrodes.
Generally, construction of an electret is performed by injecting electric charges into an insulation layer with a corona electrical discharge. However, with an electret that is manufactured by this method, there are the problems that the charge density in the insulation layer is low, and that the charge level drops due to usage over the long term, and accordingly a construction for an electret is desired that provides a higher charge density and moreover a longer working life.
And, in Japanese Laid-Open Patent Publication 2010-068643, there is disclosed a construction for forming an electret having planar comb tooth electrodes. In this construction, while applying heat to an alkali glass at a temperature that is less than the glass transition temperature, a voltage of around 500 V to around 1000 V is applied thereto, and the alkali ions in this alkali glass are thereby shifted so that a region that is depleted in alkali ions is formed.
In an article by Honzumi et al. (Honzumi, M., Ueno, A., Hagiwara, K., Suzuki, Y., Tajima, T., and Kasagi, N., “Soft-X-Ray-Charged Vertical Electrets and Its Application to Electrostatic Transducers”, Proc. 23rd IEEE Int. Conf. MEMS, Hong Kong, China, Jan. 24-28, 2010, pp. 635-638), there is disclosed a method for forming an electret by irradiating soft X-rays into between comb tooth electrodes having a three-dimensional structure, and thereby injecting electric charges generated in the atmosphere into the insulating layer for the electret. However, with this method as well, in a similar manner to the case of charge insertion by corona discharge, it is difficult to make the charge density in the insulation layer high.